Residual energy recovery in a drug delivery device

ABSTRACT

An implantable drug delivery device includes a pump motor that is driven by electrical energy from a storage capacitor. At the end of each pump delivery cycle, electrical energy stored in the pump motor is recovered and returned to the storage capacitor, so that it can be used in subsequent delivery cycles.

CROSS-REFERENCE TO RELATED APPLICATION(S)

Reference is made to co-pending application entitled “IMPLANTABLE DRUG DELIVERY DEVICE WITH PROGRAMMABLE RATE CAPACITOR CHARGE CONTROL”, Ser. No. ______, which is hereby incorporated by reference.

BACKGROUND

The present invention relates to implantable medical devices. In particular, the present invention relates to a charging of a storage capacitor from a battery and subsequently delivering stored energy from the storage capacitor to a pump motor.

Implantable drug delivery devices are used to provide patients with long-term dosage or infusion of a drug or other therapeutic agent. Implantable drug delivery devices may be categorized as either passive or active devices.

Passive drug delivery devices typically rely upon a pressurized drug reservoir to deliver the drug. The reservoir may be filled using a syringe. The drug is then delivered to the patient using force provided by the pressurized reservoir.

Active drug delivery devices include a pump or metering system to deliver the drug into the patient's system. The pump is electrically powered to deliver the drug from a reservoir through a catheter to a selected location within the patient's body. The pump typically includes a battery as its power source for both the pump and for the electronic circuitry used to control flow rate of the pump and to communicate through telemetry to an external device to allow programming of the pump.

Battery life is an important consideration for all implantable medical devices. With an implantable drug delivery device, efficiency of the driver circuitry that powers the pump motor is an important consideration. In one type of driver configuration, the pump motor is driven from electrical energy stored by a storage capacitor. The capacitor serves as a low-impedance, short-term energy reservoir to deliver sufficient power to the motor during assertion. During operation, the motor will be asserted periodically for a short period of time to provide a pulse flow of the drug, with longer period until the next assertion.

The efficiency of the driver circuitry can have an important effect on the lifetime of the battery, overall volume of the device including battery size, capacitor size, and size of the circuitry required, and on the overall cost of the device. Considerations in the efficiency of the driver include efficiency of charging the storage capacitor, and efficiency of delivering energy stored in the storage capacitor to the pump motor.

SUMMARY

An implantable drug delivery device increases energy efficiency by recovering energy at the end of each pump delivery cycle. The implantable drug delivery device includes a battery, a storage capacitor, a pump motor, a circuit for charging the storage capacitor from the battery, and a circuit for delivering electrical energy stored in the storage capacitor to the motor. At the end of a delivery cycle of the pump, electrical energy stored in the pump motor is recovered and returned to the storage capacitor for use in subsequent delivery cycles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an implantable drug delivery device with a motor control circuit that delivers electrical energy from a storage capacitor to a pump motor, recovers electrical energy stored in the pump motor at the end of a cycle, and returns the electrical energy to the storage capacitor.

FIG. 2 is a timing diagram illustrating operation of the device in FIG. 1 during motor assertion and subsequent energy recovery.

DETAILED DESCRIPTION

FIG. 1 shows implantable drug delivery device 10, which includes battery 12, pump motor 14, charging circuit 16, motor control circuit 18, driver control 20, and storage capacitor C₁. Battery 12 acts as a power source that provides all the electrical energy for operation of implantable drug delivery device 10. Motor 14 is, in one embodiment, a solenoid type pump. As shown in FIG. 1, motor 14 represents a load having a real component R_(M) and an inductive component L_(M). When motor 14 is asserted, a solenoid coil is energized, which produces an electromagnetic field causing a solenoid plunger or actuator to move. Motor 14 may also include a spring bias, which returns the actuator to its original position when the solenoid coil is no longer energized.

Motor 14 is typically asserted or energized for a relatively short time period, with a relatively long period between successive assertions. The delivery rate of the pump will depend on the period of time between successive assertions of motor 14 to produce a pump stroke. Assertion time of motor 14 may be on the order of milliseconds (e.g. 5 milliseconds) and the period between motor assertion will vary with delivery rate and may be on the order of several seconds (e.g. 3 seconds).

Motor 14 is isolated from battery 12 by a motor driver formed by charging circuit 16, motor control circuit 18, driver control 20, and storage capacitor C₁. Motor 14 is driven by energy stored in storage capacitor C₁, rather than directly from battery 12. As a result, a low impedance load presented by motor 14 is not directly connected to battery 12, and therefore does not cause a decrease or droop in battery voltage each time a motor assertion occurs. Stability of the battery voltage is important to proper functioning of the electrical circuitry of device 10.

Charging circuit 16 may take a number of different forms. In one embodiment, charging circuit 16 includes electronic switches, under the control of driver control 20, which are operated to provide improved efficiency in delivery of charging current from battery 12 to storage capacitor C₁. One example of a suitable charging circuit is shown in the previously mentioned co-pending application Ser. No. ______, which is incorporated by reference.

The delivery of current from storage capacitor C₁ to pump motor 14 is controlled by motor control circuit 18 in response to switching signals received from driver control 20. After storage capacitor C₁ has been charged, driver control 20 provides switch control signals Sw1-Sw4 to electronic switches M₁-M₄ of motor control circuit 18. As a result, pump motor 14 is asserted for a time period t_(ON) that is sufficient to drive the solenoid plunger or actuator to the end of its stroke.

In the embodiment shown in FIG. 1, switches M-M₄ of motor control circuit 18 are connected in an H-bridge configuration. Switch Sw1 is connected between capacitor terminal 30 and H-bridge node 32. Switch Sw2 is connected between capacitor terminal 30 and H-bridge node 34. Pump motor 14 is connected between H-bridge nodes 32 and 34. Switch Sw3 is connected between H-bridge node 32 and capacitor terminal 36. Similarly, switch M₄ is connected between H bridge node 34 and capacitor terminal 36.

This H-bridge configuration provides improved efficiency by delivering energy from storage capacitor C₁ to pump motor 14 during motor assertion period t_(ON) and by retrieving residual energy and returning that unused residual energy to storage capacitor C₁ at the end of each pump stroke during energy recovery period t_(DISCHARGE). Only a fraction of the electric energy delivered from storage capacitor C₁ during motor assertion period t_(ON) is required to trigger a specific action of solenoid pump motor 14. Part of the energy from storage capacitor C₁ is still stored in the magnetic field surrounding the solenoid (i.e. in conductive component L_(M)) after the mechanical action of pump motor 14 has been performed. The actual current I_(LOAD) through inductive component L_(M) cannot instantaneously change when the motor assertion period t_(ON) ends. It can, however, be redirected back to storage capacitor C₁ so that the energy is recovered.

The operation of motor control circuit 18 is further illustrated by FIG. 2, which shows the switch control signals to the gates of switches M₁-M₄, along with the load current I_(LOAD) and the motor current I_(M). At the beginning of motor assertion, switches M₁ and M₄ are turned on, while switches M₂ and M₃ are turned off. As a result, motor current I_(M) flows from storage capacitor terminal 30, through switch M₁, from node 32 through R_(M) and L_(M) to node 34, and through switch M₄ to storage capacitor terminal 36. During the assertion period t_(ON), load current I_(LOAD) equals motor current I_(M) in polarity, as shown in FIG. 2. At the end of assertion period t_(ON), switches M₁ and M₄ are opened, and switches M₂ and M₃ are closed to begin energy recovery period t_(DISCHARGE). Depending on switch timing, the diodes associated with M₂ and M₃ may turn on before the transistors of M₂ and M₃ turn on. The load current I_(LOAD) is now in the opposite direction of motor current I_(M), which causes electrical energy to be fed back to storage capacitor C₁. With switches M₂ and M₃ closed, a current path is established from storage capacitor terminal 36, through M₃ to node 32, through R_(M) and L_(M) to node 34, and through switch M₂ to storage capacitor terminal 30. When I_(LOAD) reaches zero, switches M₂ and M₃ are also opened so that pump motor 14 is isolated from storage capacitor C₁.

Because a motor control circuit 18 recovers residual energy and returns it to storage capacitor C₁, improved efficiency is achieved. This improvement can be on the order of 10%. Without the recovery of the residual energy remaining in pump motor 14 at the end of the assertion period, the energy will be dissipated in the form of heat, rather than being available for reuse.

The timing of operation of switches of M₁-M₄ is based upon an assertion period t_(ON) that will result in a full pump stroke. Driver control 20 may include sensing circuitry to detect when the end of assertion period t_(ON) (i.e. the end of the pump stroke) occurs. Alternatively, the time duration of assertion period t_(ON) may be determined empirically through testing and stored as an operating parameter of driver control 20.

Similarly, detection of a change in direction of flow of current I_(LOAD) at the end of energy recovery period t_(DISCHARGE) may be used by said driver control 20 to determine when to turn off switches M₂ and M₃. A small resistance in the current path of current I_(LOAD) or current I_(M) may be used to develop a voltage feedback signal to driver control 20.

Although an H-bridge configuration has been illustrated for motor control circuit 18, other known switching configurations may be used. The switching configuration should provide a current path at the end of the assertion period, to return energy stored in inductive component L_(M) of pump motor 14 to storage capacitor C₁.

With the present invention, implantable devices using motors that retain residual energy can be operated with smaller sized batteries, or can have a longer usable life because of the improved efficiency in transfer of energy from the storage capacitor to the motor.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. 

1. An implantable drug delivery device comprising: a battery; a storage capacitor; a solenoid pump; a charging circuit for charging the storage capacitor from the battery; and a motor control circuit for delivering electrical energy stored in the storage capacitor to the solenoid pump to produce a pump stroke and for recovering electrical energy stored in the solenoid pump at an end of the pump stroke and returning the electrical energy recovered to the storage capacitor.
 2. The device of claim 1, wherein the motor control circuit comprises an H-bridge switching circuit.
 3. The device of claim 2, wherein the H-bridge switching circuit comprises: a first switch connected between a first terminal of the storage capacitor and a first node; a second switch connected between the first terminal and a second node; a third switch connected between the first node and a second terminal of the storage capacitor; and a fourth switch connected between the second node and the second terminal of the storage capacitor.
 4. The device of claim 3, wherein the solenoid pump is connected between the first node and the second node.
 5. The device of claim 4, wherein the motor control circuit further comprises: a switch control for providing switch control signals to cause the first and fourth switches to be on and the second and third switches to be off when delivering electrical energy to the solenoid pump, and to cause the first and fourth switches to be off and the second and third switches to be on when recovering electrical energy stored in the solenoid pump and returning the electrical energy recovered to the storage capacitor.
 6. An implantable drug delivery device comprising: a battery; a electromechanical pump motor; a motor driver including a storage capacitor, the motor driver charging the storage capacitor from the battery and driving the pump motor with electrical energy from the storage capacitor, the motor driver recovering energy stored in the pump motor and returning the energy recovered to the storage capacitor.
 7. The device of claim 6, wherein the motor driver includes a switching circuit for causing current to flow in a first direction from the storage capacitor to the pump motor during a motor assertion period and causing current to flow in a second direction from the pump motor to the storage capacitor during an energy recovery period following the motor assertion period.
 8. The device of claim 1, wherein the switching circuit comprises an H-bridge switching circuit.
 9. The device of claim 8, wherein the H-bridge switching circuit comprises: a first switch connected between a first terminal of the storage capacitor and a first node; a second switch connected between the first terminal and a second node; a third switch connected between the first node and a second terminal of the storage capacitor; and a fourth switch connected between the second node and the second terminal of the storage capacitor.
 10. The device of claim 9, wherein the pump motor is connected between the first node and the second node.
 11. The device of claim 10, wherein the motor driver further comprises: a control for providing switch control signals to cause the first and fourth switches to be on and the second and third switches to be off during the motor assertion period, and to cause the first and fourth switches to be off and the second and third switches to be on during the energy recovery period.
 12. The device of claim 7, wherein current flow through the pump motor is in the same direction during both the motor assertion period and the energy recovery period.
 13. A method of operating a drug delivery device, the method comprising: charging a storage capacitor from a battery; delivering stored electrical energy from the storage capacitor to a pump motor; and recovering electrical energy from the pump motor and returning the electrical energy recovered to the storage capacitor.
 14. The method of claim 13, wherein delivering stored electrical energy comprises causing current to flow in a first direction from the storage capacitor to the pump motor during a motor assertion period; and wherein recovering electrical energy comprises causing current to flow in a second direction from the pump motor to the storage capacitor during an energy recovery period following the motor assertion period.
 15. The method of claim 14, wherein current flow through the pump motor is in the same direction during both the motor assertion period and the energy recovery period.
 16. The method of claim 15, wherein the pump motor is connected to the storage capacitor through an H-bridge switching circuit.
 17. The method of claim 16, wherein the H-bridge switching circuit comprises: a first switch connected between a first terminal of the storage capacitor and a first node; a second switch connected between the first terminal and a second node; a third switch connected between the first node and a second terminal of the storage capacitor; and a fourth switch connected between the second node and the second terminal of the storage capacitor.
 18. The method of claim 17, wherein the solenoid pump is connected between the first node and the second node.
 19. The method of claim 18, wherein causing current to flow in a first direction comprises: turning on the first and fourth switches and turning off the second and third switches during the motor assertion period.
 20. The method of claim 19, wherein causing current to flow in a second direction comprises: turning off the first and fourth switches and turning on the second and third switches during the energy recovery period. 